Southeastern Bay Of Bengal Research Articles

High spatial resolution marine gravity trend determined from multisatellite altimeter data over Bay of Bengal

SUMMARY Mass redistribution in the Earth system induce variations of the Earth's gravity field. Now, the time-varying gravity models from the Gravity Recovery and Climate Experiment (GRACE) mission can only estimate the large-scale gravity changes, so the high-resolution marine gravity trend (MGT) model is urgently required to detect small-scale Earth's mass migration. The sea level change is a significant response to marine gravity field change. Here, we propose to estimate the high-resolution MGT using the sea level trend (SLT). Firstly, the SLT model caused by marine mass change (MMC) on 5′ × 5′ grids covering the Bay of Bengal (BOB) is established based on multisatellite altimetry data and EN4 quality-controlled ocean data, named BOB_MMC_SLT. Then, the marine mass trend (MMT) is calculated using the BOB_MMC_SLT. The spherical harmonic function (SHF) method is applied to estimate MGT using the MMT, and this MGT model on 5′ × 5′ grids, named BOB_SHF_MGT, is used to study marine gravity changes and their associated geophysical processes. The results show that, the MGT mean of BOB_SHF_MGT is about 0.14 μGal yr−1, which indicates that marine gravity in BOB is rising. The earthquakes mainly occur in the southeastern BOB where MGT is obviously rising, which may be related to the increased density of the Burma Plate due to the subduction of the India Plate and the Australia Plate. BOB_SHF_MGT shows that the marine gravity rise rate is increasing from the 85°E ridge to Andaman–Nicobar ridge, with a maximum at the location where the India Plate subducts to the Burma Plate. The MGT model based on altimetry data constructed by SHF method is important for the study of small-scale mass migration near the subduction boundaries.

A Large Winter Chlorophyll‐a Bloom in the Southeastern Bay of Bengal Associated With the Extreme Indian Ocean Dipole Event in 2019

AbstractA large Chlorophyll‐a (Chla) bloom developed in the southeastern Bay of Bengal (BoB) (4°N–10°N, 88°E−95°E) during winter 2019 and was observed from Moderate Resolution Imaging Spectroradiometer‐Aqua Chla data. By analyzing various observational and reanalysis datasets, we found that remote Equatorial Indian Ocean (EIO) upwelling waves and local Ekman pumping associated with the extreme Indian Ocean Dipole (IOD) event in 2019 were responsible for the Chla bloom. The extreme IOD 2019 favored strong upwelling Kelvin waves propagating from the eastern EIO into the BoB along the eastern boundary. These Kelvin waves were reflected into the central BoB as upwelling Rossby waves manifested by eddylike sea surface height propagating westward approximately along 5°N. In addition, local wind stress curl generated strong Ekman pumping in the southeastern BoB. As a result, upwelling processes associated with westward‐propagating Rossby waves and local Ekman pumping together uplifted the thermocline, breaking the stratification so that subsurface nutrient‐rich water was transported to the surface to cause a large Chla bloom in the southeastern BoB during winter 2019. The Chla bloom had a ring structure and its spatial shape was modulated by westward upwelling Rossby waves and cyclonic eddies. This study highlights the important role of IOD events in the formation of Chla bloom in the southeastern BoB during winter.

Morphological and molecular systematics of swimming crabs (Decapoda: Brachyura: Portunidae) from India collected on-board the FORV Sagar Sampada (cruise no. 378, 385 and 392), with notes on biogeography of the Indian portunid fauna.

Morphological and molecular characters of swimming crabs (Family Portunidae Rafinesque-Schmaltz, 1815) collected during one cruise each of the Fishery Oceanographic Research Vessel Sagar Sampada in the northeastern Arabian Sea (97–110 m depth), southwestern Bay of Bengal (56 m), and southeastern Bay of Bengal (56 m) and western Andaman Sea (53 m) were studied. They are referred to 17 species representing 4 subfamilies, 8 genera and 2 subgenera. Cycloachelous levigatus Koch, 2021, Monomia rubromarginata (Lanchester, 1900), Thalamita malaccensis Gordon, 1938 are first records from Indian waters. The study also reveals variations in the pleonal and genital morphology of Monomia argentata (A Milne Edwards, 1861) form B (sensu Stephenson and Rees, 1967a). Mitochondrial Cytochrome oxidase subunit I and 16S rRNA gene sequences of 14 and 16 species, respectively revealed a monophyletic subfamily Lupocyclinae Alcock, 1899, and polyphyletic subfamilies Portuninae Rafinesque, 1815 and Thalamitinae Paulson, 1875. Analysis of the biogeographic distribution patterns of Indian portunids revealed a total of 104 species representing 22 genera, thereby suggesting mixed diversity in different ecological regions. The highest number of 72 species and 2 subspecies were recorded from the West and South Indian Shelf Province followed by Andaman Province (69 species), Bay of Bengal Province (54 species) and Central Indian Ocean Islands Province (26 species). Among the individual marine ecoregions, Andaman and Nicobar Islands recorded the highest number of species (69 species), followed by South India (57), Eastern India (50), Western India (47), Northern Bay of Bengal (32) and Maldives (26).

Redistribution of riverine and rainfall freshwater by the Bay of Bengal circulation

We use satellite-derived currents and a Lagrangian approach to investigate the redistribution of the precipitation minus evaporation (P-E) and river freshwater inputs into Bay of Bengal (BoB) by the oceanic circulation. We find a key role of Ekman transport in shaping the BoB freshwater distribution. Until September, the summer monsoon winds induce eastward Ekman transport, which maintains freshwater near its major rivers and rain sources in the northeastern BoB. The winter monsoon Ekman transport strongly contributes to the surface flow in many areas of the interior BoB. This ~ 0.15 m s−1 westward transport overcomes the weaker offshore transport by mesoscale motions and pushes a ~ 40/45% mixture of P-E and Ganges–Brahmaputra freshwater into the East Indian Coastal Current (EICC). In agreement with previous studies, we find that the EICC then transports Ganges–Brahmaputra freshwater southward, allowing the formation of a narrow freshwater tongue or “river in the sea” along the coast east of India in November. Ekman transport thus operates jointly with the EICC to allow the “river in the sea” formation. The EICC is nonetheless a “leaky pipe” as only ~ 22% of the Ganges–Brahmaputra, and ~ 9% of the P-E monsoonal freshwater inputs exit the BoB near Sri Lanka. The winter monsoon anticyclonic circulation in fact brings more rain freshwater from the equatorial Indian Ocean into the southeastern BoB than it exports freshwater through the EICC. As a result, the BoB circulation contributes to a net freshwater gain that amounts to 11% of the local rain and freshwater inputs.

Influence of the freshwater plume dynamics and the barrier layer thickness on the CO2 source and sink characteristics of the Bay of Bengal

The high influx of freshwater in the Bay of Bengal (BoB) region forms a freshwater plume at the surface. This plume's dispersal spreads a low-density layer of water on the surface, forming a barrier layer between the mixed layer depth and the isothermal layer. The low saline water reduces the sea surface partial pressure of carbon dioxide (pCO2), and the barrier layer thickness (BLT) inhibits the interaction between the subsurface and surface layers of BoB. In this study, we run a high resolution coupled physical-biogeochemical model to understand the effect of variability of freshwater plume spread (FPS) and the BLT on the seasonality of sea-surface pCO2. By comparing with observation data, we find that the model satisfactorily simulates the sea-surface pCO2 and the FPS for the BoB region. As the FPS begins to develop in the southwest monsoon season, the CO2 sink region also begins to form in the same manner. The region with sea surface salinity (SSS) less than 31 PSU was recognized as a freshwater plume. The region with SSS lower than 31 PSU showed under-saturated levels of pCO2. The model analysis showed that the CO2 sink spread exceeded the FPS in the winter monsoon season, covering almost 50% of the BoB region. This spread was attributed to the BLT, which is highest during the winter monsoon season. Based on the spatial CO2 flux, we find the Northern BoB a sink of atmospheric CO2 (−0.35±0.75 mol C m-2 yr-1). The Southwestern BoB acts as a source of CO2 (0.72±0.61 mol C m-2 yr-1) throughout the year and has the highest magnitude of flux, whereas the Southeastern BoB also acts as a weak source of CO2 (0.214±0.52 mol C m-2 yr-1).

Monthly and Spatial Distribution of Ocean Wave Parameters in the Bay of Bengal Using ERA5 Reanalysis Data

The purpose of this study is to assess the spatial distribution of ocean wave parameters, and their correlation and anomalies in the Bay of Bengal (BoB) using about 40 years of ERA5 reanalysis data. Collected data were analyzed by using geographic information system (GIS) and computer programming languages. It is revealed that higher values of SWH (2.50–2.58m) dominated the central, southern and south-western BoB during June–August. The monthly climatological mean of MSS and Cd are ranged from 0–0.013 and 0.0006–0.0016 respectively in the study area. WSP were higher (10.31–10.84m/s) in southwestern BoB during June–August. Tm is higher (11.23–11.03s) during April–September in southern and south–eastern BoB. Lower Tm (2.36–2.73s) is found in western and eastern BoB. SWH, WSP, Tm, MSS and Cd have unfolded significant impact on each other during most of the month. Strong anomaly has been reflected in the SWH, WSP, and Tm during March–April.

Submarine Groundwater Discharge from an Urban Estuary to Southeastern Bay of Bengal, India: Revealed by Trace Element Fluxes.

Submarine groundwater discharge and associated trace element fluxes from the Coleroon River estuary to south bay, India, has been attempted, because increasing trace elements could result in harmful algal blooms and eutrophication. Trace elements (Al, Cr, Mn, Fe, Ni, Cu, Zn, Sr, Mo, Ba, Pb, Th, and U) in surface water, pore, and groundwater samples were monitored for 10days in three locations (A, B, and C) by considering tidal fluctuations. The trace elements Al, Cr, Fe, Ni, Zn, Sr, Mo, Pb, Th, and U were greater and found to be influenced by processes, such as fresh groundwater discharge and seawater intrusion. Lower Mn, Cu, and Ba signifies impact due to sediment adsorption, mixing, and elemental exchange during fresh groundwater and seawater mixing. Salinity versus trace element plot infers greater trace element mobility with cumulative salinity influenced by the conformist behavior of freshwater, seawater, and mixing. The calculated submarine groundwater discharge supported dissolved trace elements fluxes were 107,047.8nmol d-1m-1 for location A, 183,520.2nmold-1m-1 for location B, and 181,474.4nmold-1m-1 for location C, respectively. Variations in dissolved trace elements fluxes are attributed to variations in pH, free redox environment in the aquifer, adsorption or desorption by sediments, and the environmental cycle of marine organisms.

Geochemistry of core sediments from the southeastern Bay of Bengal: Inferences on weathering and early diagenetic changes

A sediment core (ABP24/05), collected at a water depth of 3520 ​m from the southeastern Bay of Bengal was studied to determine the change in chemical weathering during the last glacial to deglacial periods and the factors of sedimentary environment which controlled earliest diagenetic changes in the sediment after its deposition. High ratios of K/Rb, Ti/Al and Zr/Rb during ~45 to ~18 ​cal ​kyr B.P. in the core sediments may be attributed to the stronger physical erosion and turbidity currents activity during this period. This might have brought a higher quantity of unaltered minerals to the study area. Low ratios of K/Rb, Zr/Rb, and Ti/Al and increase of SiO2/TiO2, Rb/Al and Cs/Al from ~18 ​cal ​kyr B.P. to present may be indicating an increase in the rate of chemical weathering during this period. The time of increased chemical weathering in the study area is consistent with deglaciation warming in the tropical Indian Ocean and strengthening of river runoff into the Andaman Sea. Climate change during the interglacial period by increased solar insolation thereby strengthened the summer monsoon which might have led to intensified chemical weathering in the source region since ~18 ​cal ​kyr B.P. The low organic carbon (OC), high Mn/Al, Fe/Al and the Mn-oxides minerals precipitation indicate prevailing of oxic conditions during ~11 ​cal ​kyr B.P. in the core sediments, which is contradictory to suboxic conditions developed in the deep ocean sediments in the western Bay of Bengal and the equatorial Indian Ocean. The low terrigenous influx and export of less OC to the bottom sediments might have created a favorable condition for the formation of Mn-oxides in the study area during Holocene.

Decadal Variability of the Barrier Layer and Forcing Mechanism in the Bay of Bengal

AbstractUsing Simple Ocean Data Assimilation data set for 1951–2010, we analyze the decadal variability of barrier layer thickness (BLT) and its forcing mechanisms in the Bay of Bengal (BoB). The decadal variability of BLT shows a close connection with the Pacific Decadal Oscillation (PDO). The PDO‐induced spatial distribution of BLT is similar to its first Empirical Orthogonal Function mode, showing a meridional dipolar pattern. PDO can exert an influence on the decadal variations in the Walker circulation to indirectly modulate decadal variations in BLT. Weakened southwesterly winds over the BoB associated with a positive PDO phase enhance shoreward Ekman transport along the eastern and northern boundary of the BoB and therefore deepen isothermal layer depth, which further induces positive BLT anomalies along the eastern and northern coast of the BoB. On the other hand, the weakened winds decrease oceanic heat loss; meanwhile, the enhanced upwelling driven by positive wind stress curl anomalies shoals the isothermal layer depth and thus decreases the BLT over the southeastern BoB. The opposite is true for the negative PDO phase. Using an ocean mixed layer model, we diagnose the relative contribution of different forcing terms to BLT variations on decadal timescales. It is found that horizontal advection and freshwater flux‐induced entrainment dominate the decadal BLT variability in the northern BoB, whereas heat and freshwater flux‐induced entrainment play a dominant role in the southern BoB.

Upper oceanic response to tropical cyclone Phailin in the Bay of Bengal using a coupled atmosphere-ocean model

A numerical simulation of very severe cyclonic storm ‘Phailin’, which originated in southeastern Bay of Bengal (BoB) and propagated northwestward during 10–15 October 2013, was carried out using a coupled atmosphere-ocean model. A Model Coupling Toolkit (MCT) was used to make exchanges of fluxes consistent between the atmospheric model ‘Weather Research and Forecasting’ (WRF) and ocean circulation model ‘Regional Ocean Modelling System’ (ROMS) components of the ‘Coupled Ocean-Atmosphere-Wave-Sediment Transport’ (COAWST) modelling system. The track and intensity of tropical cyclone (TC) Phailin simulated by the WRF component of the coupled model agrees well with the best-track estimates reported by the India Meteorological Department (IMD). Ocean model component (ROMS) was configured over the BoB domain; it utilized the wind stress and net surface heat fluxes from the WRF model to investigate upper oceanic response to the passage of TC Phailin. The coupled model shows pronounced sea surface cooling (2–2.5 °C) and an increase in sea surface salinity (SSS) (2–3 psu) after 06 GMT on 12 October 2013 over the northwestern BoB. Signature of this surface cooling was also observed in satellite data and buoy measurements. The oceanic mixed layer heat budget analysis reveals relative roles of different oceanic processes in controlling the mixed layer temperature over the region of observed cooling. The heat budget highlighted major contributions from horizontal advection and vertical entrainment processes in governing the mixed layer cooling (up to −0.1 °C h−1) and, thereby, reduction in sea surface temperature (SST) in the northwestern BoB during 11–12 October 2013. During the post-cyclone period, the net heat flux at surface regained its diurnal variations with a noontime peak that provided a warming tendency up to 0.05 °C h−1 in the mixed layer. Clear signatures of TC-induced upwelling are seen in vertical velocity (about 2.5 × 10−3 m s−1), rise in isotherms and isohalines along 85–88° E longitudes in the northwestern BoB. The study demonstrates that a coupled atmosphere-ocean model (WRF + ROMS) serves as a useful tool to investigate oceanic response to the passage of cyclones.

Response of upper ocean and impact of barrier layer on Sidr cyclone induced sea surface cooling

In the present study an attempt has been made to investigate the impact of salinity stratification on the SST during the tropical cyclone (TC) passage. In this context, a severe post monsoon cyclone, Sidr, (Category 4) that developed over the south-eastern Bay of Bengal (BoB) during 11–16 November, 2007 was chosen as a case study. Pre-existence of a thick barrier layer (BL), temperature inversions and a higher effective oceanic layer for cyclogenesis (EOLC) were noticed along the path of the Sidr cyclone. The analysis of available Argo floats along the Sidr cyclone track also revealed less cooling during as well as after its passage as was reported from satellite derived SST. The role of BL on Sidr induced sea surface cooling was investigated using a diagnostic mixed layer model. Model results also depict the reduced sea surface cooling during the passage of Sidr. This is attributed to the presence of BL which results in the inhibition of the entrainment of cool thermocline water into the shallow mixed layer. Climatological as well as in situ observations of tropical cyclone heat potential (TCHP) and EOLC shows that the Sidr cyclone propagated towards the regions of higher EOLC.

Genesis of the South Asian High and Its Impact on the Asian Summer Monsoon Onset

AbstractThe formation of the South Asian high (SAH) in spring and its impacts on the Asian summer monsoon onset are studied using daily 40-yr ECMWF Re-Analysis (ERA-40) data together with a climate-mean composite technique and potential vorticity–diabatic heating (PV–Q) analysis. Results demonstrate that, about 2 weeks before the Asian summer monsoon onset, a burst of convection over the southern Philippines produces a negative vorticity source to its north. The SAH in the upper troposphere over the South China Sea is then generated as an atmospheric response to this negative vorticity forcing with the streamline field manifesting a Gill-type pattern. Afterward, the persistent rainfall over the northern Indochinese peninsula causes the SAH to move westward toward the peninsula. Consequently, a trumpet-shaped flow field is formed to its southwest, resulting in divergence pumping and atmospheric ascent just over the southeastern Bay of Bengal (BOB).Near the surface, as a surface anticyclone is formed over the northern BOB, an SST warm pool is generated in the central–eastern BOB. This, together with SAH pumping, triggers the formation of a monsoon onset vortex (MOV) with strong surface southwesterly developed over the BOB. Enhanced air–sea interaction promotes the further development and northward migration of the MOV. Consequently, the wintertime zonal-orientated subtropical anticyclone belt in the lower troposphere splits, abundant water vapor is transported directly from the BOB to the subtropical continent, and heavy rainfall ensues; the atmospheric circulation changes from winter to summer conditions over the BOB and Asian summer monsoon onset occurs.

Episodic phytoplankton bloom events in the Bay of Bengal triggered by multiple forcings

Two episodic phytoplankton bloom events were identified in the southeastern and southwestern Bay of Bengal in January 1998 and December 2005, respectively, from SeaWiFS-derived chlorophyll-a concentration data collected between September 1997 and December 2010. Possible causes were examined using time series datasets of satellite-derived sea surface height anomalies, sea surface temperature, wind stress and Ekman pumping velocity data. It was found that strong cyclonic eddies with long residence time transported cold, nutrient-rich water from the subsurface layer to the euphotic zone, and were responsible for sustaining the continuous growth of phytoplankton that led to the two aforementioned phytoplankton blooms. Moreover, regarding the event in December 2005, three tropical cyclones passed through the region of the phytoplankton bloom and could have been additional factors that enhanced the bloom. Regarding the event in January 1998, transportation from the adjacent Andaman Sea by the geostrophic current and an anomalous zonal easterly wind contributed to the phytoplankton bloom in the southeastern Bay of Bengal. In addition, both blooms occurred during Indian Ocean Dipole (IOD) years. The absence of the second downwelling Kelvin waves in positive IOD year of 1997–1998 favouring the long residence of the strong cyclonic eddy in the eastern bay, and the strong wind-induced mixing in negative IOD year of 2005 might contribute to enhancing productivity.

Radiative properties of Bay of Bengal aerosols: Spatial distinctiveness and source impacts

Simultaneous and collocated measurements of spectrally resolved scattering and absorption coefficients and mass concentration of near‐surface composite aerosols in the marine atmosphere over the Bay of Bengal (BOB), along with incoming shortwave (0.3–3 μm) global solar radiation and columnar spectral aerosol optical depths (AOD), were made on a research cruise during the winter phase of the Integrated Campaign for Aerosols, Gases and Radiation Budget (W‐ICARB). The aerosol radiative properties revealed distinct spatial features associated with the contrasting outflows from Indo‐Gangetic Plain (IGP) and East Asia. Both scattering and absorption coefficients depicted very high values (>200 and >15 Mm−1) over the northwestern and southeastern BOB and extremely low values (<50 and <10 Mm−1) over the central BOB. The mean value of the total scattering coefficient at 550 nm (∼123.7 ± 85.3 Mm−1) over the entire BOB during winter was higher than the mean values (∼94 ± 47 Mm−1) reported for the premonsoon season. While SSA at 550 nm showed very low values (<0.8) over a very large region in the central BOB and moderately low values over the southern BOB (∼0.85–0.9), the columnar AOD varied from the least values of ∼0.1 over the northeastern BOB to the highest values of ∼0.8 over the northwestern BOB. While significant cooling was observed at the top of the atmosphere and surface over the northwestern BOB, the atmospheric forcing was found to be significantly high (∼15 W m−2) over the southern BOB, where the aerosol radiative forcing efficiency (ARFE) at the surface was also found to be high. Examination of the Moderate Resolution Imaging Spectroradiometer (MODIS)‐derived fire count along with the advection pathways revealed a strong contribution from the emissions of biomass smoke from East Asia, which might be contributing to the enhanced aerosol induced warming over the southern BOB. However, the ARFE at the surface was low over the northwestern BOB, where the advection from IGP was predominant.

Optical and Physical Properties of Atmospheric Aerosols over the Bay of Bengal during ICARB

Abstract Simultaneous and collocated measurements of total and hemispherical backscattering coefficients (σ and β, respectively) at three wavelengths, mass size distributions, and columnar spectral aerosol optical depth (AOD) were made onboard an extensive cruise experiment covering, for the first time, the entire Bay of Bengal (BoB) and northern Indian Ocean. The results are synthesized to understand the optical properties of aerosols in the marine atmospheric boundary layer and their dependence on the size distribution. The observations revealed distinct spatial and spectral variations of all the aerosol parameters over the BoB and the presence of strong latitudinal gradients. The size distributions varied spatially, with the majority of accumulation modes decreasing from north to south. The scattering coefficient decreased from very high values (resembling those reported for continental/urban locations) in the northern BoB to very low values seen over near-pristine environments in the southeastern BoB. The average mass scattering efficiency of BoB aerosols was found to be 2.66 ± 0.1 m2 g−1 at 550 nm. The spectral dependence of columnar AOD deviated significantly from that of the scattering coefficients in the northern BoB, implying vertical heterogeneity in the aerosol type in that region. However, a more homogeneous scenario was observed in the southern BoB. Simultaneous lidar and in situ measurements onboard an aircraft over the ocean revealed the presence of elevated aerosol layers of enhanced extinction at altitudes of 1 to 3 km with an offshore extent of a few hundred kilometers. Back-trajectory analyses showed these layers to be associated with advection from west Asia and western India. The large spatial variations and vertical heterogeneity in aerosol properties, revealed by the present study, need to be included in the regional radiative forcing over the Bay of Bengal.

Simulation of Synoptic- and Subsynoptic-Scale Phenomena Associated with the East Asian Summer Monsoon Using a High-Resolution GCM

Abstract A 20-yr simulation using a global atmospheric general circulation model with a resolution of 0.5° latitude × 0.625° longitude is compared with observational findings. The primary goal of this survey is to assess the model performance in reproducing various summertime phenomena related to the continental-scale Asian monsoon in general, and the regional-scale East Asian monsoon in particular. In both model and observed atmospheres, the seasonal march of the precipitation centers associated with the Asian summer monsoon is characterized by onsets occurring earliest over the southeastern Bay of Bengal, followed by rapid northeastward advances over Indochina, the South China Sea–Philippine Sea and the western Pacific, northward evolution in the East Asian sector, as well as northwestward development over the Bay of Bengal, the Indian subcontinent, and the Arabian Sea. This onset sequence is accompanied by southwesterly low-level flows over the rainy regions, as well as northwestward migration of the 200-mb Tibetan anticyclone. Analysis of the heat sources and sinks in various regions illustrates the prominent role of condensational heating in the local energy budget during the mature phases of monsoon development. In accord with observations, the simulated monsoon rains in the East Asian sector are organized about zonally elongated “mei-yu–baiu” (plum rain) systems. These precipitation features advance to higher latitudes during the June–July period, in conjunction with displacements of the axis of the low-level anticyclone over the subtropical western Pacific. A detailed case study is performed on a prominent rainy episode in the simulation. The model is capable of reproducing the observed intense gradients in temperature, humidity, and moist static stability in the vicinity of the mei-yu–baiu front, as well as the spatial relationships between the rainband and the three-dimensional flow field. The axis of the mei-yu–baiu rainband in this event is aligned with the trajectory of a succession of mesoscale cyclonic vortices, which originate from southwestern China and travel northeastward over the Yangtze River basin.

Characteristics, evolution and mechanisms of the summer monsoon onset over Southeast Asia

AbstractBased on the 1979–95 mean pentad reanalysis data from the US National Centers for Environmental Prediction, the climatological characteristics and physical mechanism of the Asian summer monsoon (ASM) onset are investigated. Special focus is given to whether the ASM onset starts earlier over the Indochina Peninsula than over the South China Sea (SCS) and why the ASM is established the earliest over Southeast Asia.An examination of the composite thermodynamic and dynamic quantities confirms that the ASM onset commences earliest over the Indochina Peninsula, as highlighted by active convection and rainfall resulting from the convergence of southwesterly flow from the Bay of Bengal (BOB) vortex and easterly winds associated with the subtropical anticyclone over the SCS. Two other important characteristics not previously noted are also identified: the earliest reversal of meridional temperature gradient throughout the entire troposphere and the corresponding establishment of an easterly vertical wind shear, which are due to upper level warming caused by eddy (convective) transport of latent heat.These changes in the large‐scale circulation suggest that, in addition to rainfall, a reversal in the planetary‐scale circulation should be included in determining the timing of the ASM onset. With such a consideration, the climatological ASM onset occurs first over southeastern BOB and southwestern Indochina Peninsula in early May, and then advances northeastward to reach the SCS by the fourth pentad of May (16–20 May). The monsoon then covers the entire Southeast Asia region by the end of May. Subsequently, a similar onset process begins over the eastern Arabian Sea, India and western BOB, and the complete establishment of the ASM over India is accomplished in mid June. In the process of the onset of each ASM component, the reversal of the upper level planetary‐scale circulation depends strongly on that of the meridional temperature gradient. Over the Indochina Peninsula, the seasonal transition of upper level temperature results from convection‐induced diabatic heating, whereas over western Asia it is attributed to subsidence warming induced by the active ascending motion over the former region.The steady increase in surface sensible heating over the Indian subcontinent and the latent heating over the tropical Indian Ocean in April to early May appear to be the major impetus for the development of the cyclonic vortex over the BOB. A similar enhancement over the Arabian Peninsula and the surrounding regions is also identified to be crucial to the development of the so‐called onset vortex over the Arabian Sea, and then ultimately to the ASM onset over India. Copyright © 2004 Royal Meteorological Society